Activation of ferroelectrics

ABSTRACT

776,568. Ferroelectric materials. AMERICAN LAVA CORPORATION. Dec. 9, 1954 [Dec. 11, 1953], No. 35714/54. Class 37. [Also in Group XL (b)] A method of activating a ferroelectric ceramic comprises (i) applying an activating current to the ceramic with the ceramic heated to not more than 10‹ C. above its transition temperature and (ii) continuing to supply the activating current whilst allowing the ceramic to cool. The term &#34; activated &#34; means treatment to produce increased dielectric constant. The term &#34; transition temperature &#34; means the temperature at which the first crystal transformation above room temperature is effected. The current is D.C. and is preferably that which produces a change of at least 15 per cent and more preferably the maximum change in dielectric constant. The current may be maintained. substantially constant while the ceramic cools to at least 65‹ C. below its transition temperature. The maximum change in dielectric constant for a particular ceramic is found by using sample pieces of the material and subjecting each piece to activation until that current is found which produces the largest. change in dielectric constant.  Detailed embodiment.-A ceramic cylinder 10, shown in Fig. 1, has silver electrode coatings 11, 12 and leads 13,14. The ceramic material comprises 96 per cent barium titanate and 4 per cent lead titanate. A number of such cylinders are immersed in chlorinated diphenyl and slowly heated to 10‹ C. above the transition temperature (120‹ C). A D.C. is then applied to leads 13, 14 and increased gradually until it reaches 14 micro-amperes per square inch of electrode surface. After a short time the temperature is lowered slowly and the current maintained constant by increasing the applied voltage as the cylinders cool to about 65‹ C. below the transition temperature. After complete cooling the diphenyl is removed by dipping the cylinders in acetone. The activating current value of 14 micro-amperes is that which produces the greatest change in dielectric constant for the given ceramic. This value is found by treating groups of cylinders using different current values for each group and measuring the capacity values and compiling results. Fig. 1 has application as a condenser or piezo-electric device. Ceramic materials.-The invention may be performed using the following ferro-electric ceramics : titanates of alkali metals and alkaline earth metals; tungstates, bismuth oxides, strontium cerate, boron phosphate, arsenic sesquioxide, lithium ferrite, lithium sulphate and antimony iodide; also titanate compositions comprising selectively as additives or minor proportions the stannates and zirconates of barium, lead and calcium and the titanates of calcium, strontium and lead.

Nov. 22, 1955 J. D. WALLACE ACTIVATION OF FERROELECTRICS Filed Dec. 11,1955 1 5 20 CURRENT (MICROAMPERES) INVENTOR JOHN D. WALLACE Fig. 3

.F. M A TTOR NE Y5 United States Patent- Ofiiice 2,724,171 Patented Nov.22, 1955 2.724.171 ACTIVATION F FERRGELECTRIQS John D, Wallace, Qreland,Pa.

Application December 11, 1953, Serial No. 397,801

4 C a ms! 2 -35) (Granted under Title 35, U, S. Code (1952), See. 266)The invention described herein may be manufactured and used by or'forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to a new method for activating ferroelectrics toimpart to them electromechanical properties and to the product producedby the method,

Broadly, the process of the invention comprises sub: jectingferroelectric materials to an external, electric field to producematerials of increased dielectric constant and having enhancedelectromechanical properties. This procedure is referred to herein asactivating the ferroelectric.

Previous methods directed to activating ferroeiectrics are based on thepremise that the effect produce is due to a crystal realignment whichmay be effected by application of a voltage stress. For example, oneprior art method comprises activating titanate ceramics by theapplication of high voltages at room temperature; and another methodcomprises the activation of barium titanate ceramic by the applicationof a voltage at the transition point of the material, both methodsapparent- 1y based on the theory that a crystal realignment is effectedby the voltage stress produced. These and other conventional methods areindicative of the fact that current criterion has apparently never beenconsidered a factor in activating ferroclectrics.

Former methods for activating ferroelectrics are subject to thedisadvantages that they do not produce a uniform'ly activated product ora completely activated product, due, mainly to the fact that without aconsideration of current criteria there is no basis for control of thedegree of activation of the material. Incomplete or non-uniformactivation results in a relatively small increase in dielectric constantin the material as well as inferior electromechanical properties. Formermethods lack reproducibility and also leave certain improvements to bedesired in the product, such as higher coupling coefficient and improvedhysteresis properties.

it is therefore an object of this invention to provide an improvedmethod for activating ferroelectrics.

It is another object of the invention to provide a method for activatingferroelectric materials which is reproducible and by which completeactivation may be obtained.

Itis a further object of this invention to provide an activatedferroelectric product for use as an electromechanical element havingenhanced electromechanical properties, an increased dielectric constant,a high coupling coefiicient and improved hysteresis characteristics Ithas'been found that the above and other objects are accomplished in thepreferred modification of the invention by the application of an optimumactivation current to ferroelectric material at about its transitiontemperature, and maintaining this current substantially constan as thematerial cools, preferablyto a point at least 65 below the transitiontemperature.

In the accompanying drawings which are incorporated as a part of thisapplication as an aid in illustrating the application of the inventionto titanate ceramic,

Fig. 1 is an isometric View of a cylindrical transducer of bariumtitanate ceramic; v

Fig. 2 is a graph illustrating the percent increase in dl". electricconstant in barium titanate ceramic elements re: sulting from activationby the present process; I

Fig. 3 shows a sampling of hysteresis loops made from data obtained ontranducers activated by the method of the invention, and I 4 Fig. 4shows a sampling of hysteresis loops made from data obtained ontranducers activated by a former volt: age method.

The invention is illustrated in the specification and drawings by itsapplication to barium titanate base ceramic, however, it is equallyapplicable to any ferro; electric ceramic, and particularly to titanateceramics. Examples of other suitable materials are, titanates of othermetals such as the alkali metals and other alkaline earth metals,niobates, and tantalates of alkali and alira: line earth metals andmixtures of all of the above. Addi: tional specific materials to whichthe invention is appli: cable are, tungstenatcs, bismuth oxides,strontium cerate, boron phosphate, arsenic sesquioxide, lithium ferrite,lithium sulfate and antimony iodide. The ceramics to which the inventionapplies may comprise of a single ferroelectric or mixtures offerroelectrics, to, which ls m y b dde a mi r perc tag o v o s teorelectric additives for tailoring specific properties. Ex; amples ofthese additives are calcium titanate, calcium zirconate, strontiumtitanate, lead titanate, barium stan: mate and combinations thereof. 7

For h pu p of h pplicati n he tran iti n can perature is the temperatureat which. the first crsytai transformation above room temperature iseffected. In the case of barium titanate, for example, it is thetempera: ture at which the crystal structure changes from tetrag na t ics u e n the spe ific i nd, c aims the transition temperature of theceramic is to be inter: mat d to e h rans ti emperatu of the f reelectric crystals contained in the ceramic. To ensure completetransformation, the material being activated is usua y eated to ab t 0C- c' e t e trans t n e n: peraturc. This temperature is conventionallydeterm ed for y material by Peakin the d e t i c nst nt at the material.s

The op m a t o current ha b en ound is be that amount of current whicheffects the greatest change in dielectric constant between unactivatedand activated material, and is a function of the cross sectional area.oi the material taken normal to the direction of polarization. It hasbeen discovered that the percentage change in dielectric constant of aferroelectric material upon activation is proportional to the degree ofactivation pro: duced in the material. This relationship between'thechange in dielectric constant and the degree of activae tion of amaterial is an'important factor in the invention.

Activating current is necessarily expressed in terms of current persquare inch ofelectrode surface for the particular material. The optimumactivation current is con veniently obtained by maximizing the change indielectric constant through activation using sample pieces of thematerial which have been electroded. That is, the material is subjectedto the present process using various amounts of current until thatamount of current is found which produces the largest change indielectric constant.

It is believed that the reason for the effectiveness of the instantmethod is as follows: It is well established that the desiredanisotrophy of barium titanate is QCCQITI? plished by the permanentdisplacement of the central titanium atom in the crystal lattice in thedirection of oxygen atoms. According to the present invention,sufficient energy is provided throughout activation to hold thepotential energy of the titanium atom at a constant value correspondingto a position near one of the oxygen atoms in the lattice. This requiresfurnishing enough energy to move the atom to the new position plusadditional energy at a rate which equals its loss in thermal energy.This energy total can be calculated. Accordingly, the present process isbased on the supplying of energy in the form of direct current to theelement being activated. The amount of current, of course, depends onthe size of the material being activated and the degree of activationrequired. In the preferred embodiment the current is maintained constantduring the process by raising the voltage as the resistance increaseswith decrease in temperature of the material.

As an aid in illustrating the invention reference is made to Fig. 1. Theceramic cylinder was made by slip casting a composition of bariumtitanate, water and defiocculant and firing the formed casting tovitrification. However, various conventional processes may be used formaking the ferroelectric ceramic elements. The electroded surfaces 11and 12 are of glass base liquid silver and were applied by spraying onthe silver and firing to approximately 1500 F. Leads 13 and 14 were thensoldered on opposite electroded surfaces as shown using a 2% silversolder.

Activation of cylinders to produce transducers was accomplished asfollows: a number of cylinders of barium titanate base ceramic wereimmersed in chlorinated biphenyl, a high resistance liquid, and slowlyheated to a temperature about 10 C. above the transition temperature,this temperature being about 120 C. for barium titanate. Using an RA-38rectifier power supply furnishing a maximum voltage and current, thecurrent was applied through the leads and increased gradually until itreached 14 microamperes per square inch of electrode surface, theoptimum amount of current for complete activation. This figure wasobtained as explained above by maximizing the change in dielectricconstant using samples of the material. After a short period, thetemperature was lowered slowly and the current maintained constant byincreasing the voltage as the pieces cooled to a temperature about 65 C.below the transition temperature. The current may be maintainedsubstantially constant until the piece has cooled to room temperature,however, the above temperature is ordinarily an adequate minimum. Aftercooling, the biphenyl was removed from the pieces by dipping in acetone.

In operation as a transducer, acoustic waves or other mechanical stimulistriking the cylinder 10 result in the generation of a voltage which istaken off on leads such as those shown at 13 and 14. The device shown inFig. 1 may also serve as a condenser when properly utilized.

The following tabulation of data obtained during activation of a batchof eight barium titanate cylindrical transducers was selected from alarger amount of similar data to illustrate the operation of theinvention. The total current required for the group is shown in thetable, a total current of 1.3 milliamperes giving 14 microamperes persquare inch of total electrode surface. Activation was conducted over aperiod of about 90 minutes.

Table I Total Current (milliamperes) Temperature (degrees F.)

The elements used in obtaining the above data showed an average changein dielectric constant of about 19% as a result of the treatment.Activated material gave a coupling coefiicient of 25 percent.

Groups of barium titanate ceramic elements were activated by the processof this invention using difierent current values for each group, theiraverage change in dielectric constant noted and the results tabulated inTable II below. The graph of Fig. 2 was made from the results shown inthe table. The elements were cylindrical in shape and the outsidesurfaces were completely electroded with the exception of the endportions. The ceramic pieces had a composition of 96% barium titanateand 4% lead titanate. They had a height of 1 /2 inches, an averageoutside diameter of 1% inches and an average thickness of inch. Thedielectric constant was ascertained as follows: A test bridge was usedfor obtaining the capacity values. The dielectric constant (k) wascalculated using the following formula. For tubes:

height in inches. Groups 1, 2, 4 and 5 contained eight elements each andgroup 3 contained 36 elements.

Table 11 Percent Current change in Group (microdielectric amperes)constant (Average) The final dielectric constant is somewhat dependentupon the purity of the material, however, the percent change indielectric constant does not vary appreciably with materials meetingreasonable purity standards.

The results in the above table show that the most effective current fortitanate ceramics is a current of 14 microamperes per square inch ofelectrode surface. Use of this current in the process was found toproduce complete activation as evidenced by the graph. However, othercurrent values are shown to be effective, and particularly the rangefrom 7.5 to 20 microamperes. The optimum activation current of 14microamperes per square inch of electrode surface applies to bariumtitanate ceramics containing PbTiOa, CaTiOs, and the stannates andzirconates of barium, lead and calcium as additives. The invention,including the range of current values, is applicable to all ceramicshaving the perovskite type crystal structure. As the results indicate,the invention is not limited in its application to the optimumactivation current alone, but in its broadest aspect includes hteapplication of quanta of current, i. e., activating currents toferroelectrics under the stated conditions to effect activa tion throughan intra-crystal rearrangement, as contradistinguished from priormethods which were based on the application of voltage directed toeffecting a change in crystal orientation through stress.

Neither is the invention in its broadest sense restricted to maintainingthe initial current constant during cooling, but it includes varying thecurrent as the material cools, the requirement being that a substantialquantum of current, in the order of several microamperes, be suppliedthroughout the cooling period.

For purposes of comparison, elements of barium titanate ceramic similarto those used to obtain the results shown above were subjected to astandard voltage activation process, i. e., they were subjected tovarious voltages at the transition temperature and the voltage Table 111Percent change in dielectric constant Voltage A comparison of theresults of the standard voltage method and the current method of thisinvention shows the effectiveness of the present method, particularlywhen the optimum activation current is used. Further, the resultsconstitute some evidence that activation is an energy phenomenonaccompanied by a change in intra-crystal structure rather than a stressphenomenon accompanied by a realignment of crystals.

The method was found to be highly reproducible, a factor resulting fromthe use of current rather than voltage criterion as a basis for controlof the degree of activation of the material.

For a further comparison of the effectiveness of the two methods,reference is made to Fig. 3 showing hysteresis loops produced by piecesactivated by the method of this invention and to Fig. 4 showinghysteresis loops produced by pieces activated by the conventionalvoltage" method. The hysteresis loops were observed using anoscilloscope with a standard circuit. The observations recorded in Figs.3 and 4 were made on the same type of elements for which results areshown in Tables I and II. The elements used for the loops of Fig. 3 weresubjected to a drive of 4,000 and 5,000 volts respectively and those ofFig. 4 were subjected to a drive of 2,000 and 2,500 volts respectively.The area under the loops shown in Fig. 3 is quite small as compared tothat under the loops of Fig. 4 even though the loops of Fig. 3 wereproduced with twice the drive of those of Fig. 4, indicating asignificant difierence in power loss to hysteresis in the piecesactivated by the two methods.

The product of the invention as illustrated in Fig. 1, when properlyutilized, may serve in various applications other than transducers, suchas, frequency control devices, electromechanical filters, supersonicsound genera tors, microphones, telephone receivers, phonograph pickups,piezoelectric relays and similar devices. evident from the propertiesset forth above.

The method of the invention provides a product highly suitable forelectromechanical applications as well as for applications based on achange in dielectric constant of the material. The method ensures a highdegree of activation as well as favorable hysteresis properties in theproduct, and is reproducible.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. it is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. The method of activating a ferroelectric ceramic which comprisesapplying electrodes to opposite sides of the ceramic, applying to theceramic at approximately its transition temperature direct currentwithin a range of about 7 to about 20 microamperes per square inch ofelectrode surface, and maintaining the supply of current substantiallyconstant until the ceramic cools.

2. The method of activating a ferroelectric ceramic which comprisesapplying electrodes to opposite sides of the ceramic, applying to theceramic at its transition temperature about 14 microamperes of directcurrent per square inch of electrode surface, and maintaining the supplyof current substantially constant as the ceramic cools.

3. The method of making an electromechanical element which comprisessubjecting a ferroelectric ceramic to an activating current atapproximately the transition temperature of the ceramic and maintainingthe activating current substantially constant until the ceramic cools.

4. The method of making an electromechanical element which comprisessubjecting a ferroelectric ceramic to an activating current atapproximately the transition temperature of the ceramic and maintainingthe activating current substantially constant until the ferroclectricceramic cools to about 65 degrees centigrade below its transitiontemperature.

This is Gray Nov. 1, 1949 Cherry Jan. 16, 1951

1. THE METHOD OF ACTIVATING A FERROELECTRIC CERAMIC WHICH COMPRISESAPPLYING ELECTRODES TO OPPOSITE SIDES OF THE CERAMIC, APPLYING TO THECERAMIC AT APPROXIMATELY ITS TRANSITION TEMPERATURE DIRECT CURRENTWITHIN A RANGE OF ABOUT 7 TO ABOUT 20 MICROAMPERES PER SQUARE INCH OFELECTRODE SURFACE, AND MAINTAINING THE SUPPLY OF CURRENT SUBSTANTIALLYCONSTANT UNTIL THE CERAMIC COOLS.